Method for incipient failure detection in electric machines

Abstract

An improved method for detecting an incipient failure in a multiphase electric motor includes the following steps: periodically measuring or continuously monitoring voltage and current values at each input to the motor; determining negative sequence voltage and current values for each periodic measured input voltage and current value; calculating an effective negative sequence impedance phasor value angle from each of the determined negative sequence voltages and current values; and comparing the calculated negative sequence impedance phasor angles and/or real and imaginary components over a plurality of periodic measurements to detect a change therein, which change is indicative of an incipient failure mode.

We claim: 
     
       1. A method for detecting an incipient failure in a multiphase electric motor, said motor having an input for each phase from a supply source, said method comprising the steps of: (a) performing a plurality of successive measurements of simultaneous voltage and current values at each said input;   (b) determining negative sequence voltage and current vector values for a plurality of said measured voltage and current values;   (c) calculating effective negative sequence impedance phasor angles from said determined negative sequence voltage and current vector values; and   (d) comparing a plurality of said effective negative sequence impedance phasor angles to detect a change therein as indicative of said incipient failure.   
     
     
       2. The method as recited in claim 1, wherein said supply source provides three phases, said phases arbitrarily designated A, B, and C. 
     
     
       3. The method as recited in claim 2, wherein each negative sequence current and voltage vector value is determined from the following relations: ##EQU5## 
     
     
       4. The method as recited in claim 3, wherein said negative sequence impedance phasor angle θ a2  (eff) is derived as follows: ##EQU6## 
     
     
       5. The method as recited in claim 3, wherein each said negative sequence impedance vector value is derived from a ratio of negative sequence voltage to negative sequence current vector values, whereby supply source variations are negated. 
     
     
       6. The method as recited in claim 5 wherein an incipient failure is indicated only when said negative sequence phasor angle changes by more than a predetermined limit. 
     
     
       7. A method for detecting an incipient failure in a multiphase electric motor, said motor having an input for each phase from a supply source, said method comprising the steps of: (a) performing a plurality of successive measurements of simultaneous voltage and current values at each said input;   (b) determining negative sequence voltage and current vector values for a plurality of said measured voltage and current values;   (c) calculating effective negative sequence impedance vector values from said determined negative sequence voltage and current vector values; and   (d) comparing like components of said effective negative sequence impedance vector values over a plurality of measurements to detect a change therein as indicative of said incipient failure.   
     
     
       8. The method as recited in claim 7 wherein step (d) compares real components of said effective negative sequence impedance vector values. 
     
     
       9. The method as recited in claim 7 wherein step (d) compares imaginary components of said negative sequence impedance vector values.